1. Field of the Invention
The present invention relates to an optical disk reading device, and in particular, to a control method for reducing the stable rotation speed of an automatic ball balancing system. In particular, the present invention pertains to a method that is applied to the rotor mechanism of an optical disk reading device to reduce the amount of vibration before the rotor mechanism reaches the working rotation speed, so as to effectively improve the service life of the bearing of the rotor.
2. Description of the Prior Art
General optical disk reading devices, such as CD-ROM, DVD-ROM, CD-RW, DVD-RAM, and other optical data reproducing or recording devices, have been widely used in multimedia computer systems and have become an important component among the peripheral devices of computer systems.
The data reading or storing speed of a conventional optical disk drive is expressed as multiplication speed (i.e., 1 multiplication speed=150 kbyte/sec). The magnitude of the multiplication speed is directly related to the spindle motor that drives the optical disk inside the optical disk drive. The faster the spindle motor rotates, the higher the multiplication speed for the reading or storing operation in the optical disk. However, when the spindle motor rotates at high speeds, the centrifugal deviation force generated by unbalance of the optical disk is also increased, which will lead to vibration, noise, and other problems. Also, excessive vibration will lead to an out-of-focus optical reading head and other unstable situations. Consequently, in order to effectively suppress vibration to ensure that the data stored in the optical disk can be read correctly by the optical disk drive, optical disk drive manufacturers have developed a type of automatic ball balancing system that functions to reduce vibration. As described in greater detail below, the automatic ball balancing system has a track on which one or more balls are disposed for movement.
The theory for the balls of the above-mentioned automatic ball balancing system to reach the desired balanced positions is based on the theory of rotor dynamics. FIGS. 1A–1C illustrate three possible conditions for the balls 2 in the automatic ball balancing system. First, when the stable rotation speed of the spindle motor is lower than the unstable critical rotation speed (called natural frequency of the suspending system), the unbalance amount of the balls 2 and the imbalance center of mass of the disk 10 of the system are in the same phase state (as shown in FIG. 1A). Second, when the stable rotation speed of the spindle motor is equal to the unstable critical rotation speed, there is a phase difference of 90° between the unbalance amount of the balls 2 and the imbalance center of mass of the disk 10 of the system (as shown in FIG. 1B). The numeral 20 in FIG. 1B illustrates the position of the balls 2 during this condition. Third, when the stable rotation speed of the spindle motor is higher than the unstable critical rotation speed, there is a phase difference of 180° between the unbalance amount of the balls 2 and the imbalance center of mass of the disk 10 of the system (as shown in FIG. 1C). Again, the numeral 20 in FIG. 1C illustrates the position of the balls 2 during this condition.
Here, the unstable critical rotation speed wc is the rotation speed at which the balls 2 are stable. As shown in FIG. 2, when the spindle motor rotates under a uniform acceleration (that is, the initial acceleration of α1) and the speed increases above the unstable critical rotation speed wc, the center of mass of the balls 2 moves to the position with the lowest potential energy. When the spindle motor reaches a stable rotation speed ws, reducing the acceleration of the motor can stabilize the balls 2 within the shortest period of time. After the balls 2 are stabilized, the balls 2 will have no relative movement with respect to the track. At that time, the spindle motor can be accelerated again to working rotation speed w. Consequently, when the spindle motor reaches the stable rotation speed ws, the rotor mechanism of the optical disk drive can be balanced automatically.
In the application of the automatic ball balancing system to the rotor mechanism, if the spindle motor is operated under a uniform acceleration to reach the working rotation speed w directly, it is still possible to reduce the unbalance amount of the rotor. However, under this condition, the rotation speed of the motor will be very high when the balls 2 are balanced. This will eventually result in the rotor being accelerated slowly and the force exerted on the rotation shaft being increased, thereby reducing the service life of the rotation shaft and the bearing.
Thus, there still remains a need to reduce the vibration in the automatic ball balancing system of an optical disk drive before the rotor mechanism reaches the working rotation speed so as to effectively improve the service life of the rotation shaft and the bearing.